1. Field of the Invention
This invention relates to a process for producing an impact-resistant polyacetal resin composition which is conveniently used as a material for such parts as clips, springs, gears, bearings, cams, levers and the like in electrical and electronic machines, office machines, automobiles, industrial machines, toys and/or sports goods, etc.
The invention also relates to a process for producing an impact-resistant polyacetal resin composition which can give parts having excellent adhesion of weld portions when the parts molded by injection molding have the weld portions.
The invention is further directed to a process for producing an impact-resistant polyacetal resin composition which can give the above-described parts having excellent outer appearance by means of melt molding.
The invention is further directed to a process for producing an impact-resistant polyacetal resin composition of which change in melt viscosity by elapse of time is small.
2. Prior Arts
Polyacetal has found wide usage as an engineering plastic excelling in mechanical properties, electrical properties and chemical properties such as chemical resistance. Its usages are limited, however, because of its inferior impact resistance compared to that of other engineering plastics. As a method to improve impact resistance, i.e., impact strength, of polyacetal, a means to blend a thermoplastic polyurethane elastomer with polyacetal has been proposed and has been already put to practical use.
For example, U.K. Patent 1,017,244 discloses a thermoplastically deformable composition, which comprises a polyurethane of high molecular weight at least 8,000 and a polyoxymethylene of molecular weight at least 4,000, the ratio of the polyurethane to the polyoxymethylene being from 5:95% by weight to 95:5% by weight.
However, a molded article obtained by melt molding a composition comprising a polyacetal and a thermoplastic polyurethane has substantially poor outer appearance. The reason for this is that each of the polymers forms an independent layer in the inside of the molded article and on the surface thereof because of poor compatibility between the two polymers, and these layers are separated from each other on the surface of the molded article.
Furthermore, when a molded article molded by injection molding has a weld portion, the adhesion of the weld portion is not always sufficient.
Under the situation described above, use of a composition which comprises a polyacetal and a thermoplastic polyurethane as a material for producing parts having complex shapes formed by weld portions and parts whose outer appearance is of a great importance has been avoided in many cases.
European Laid-open Patent No. 0565304A1 proposes a process for producing an impact-resistant polyacetal resin composition, which comprises mixing with shear a polyacetal resin (A), a thermoplastic polyurethane (B), a polyhydric alcohol (C) having at least three hydroxyl groups in the molecule, and at least one compound (D) selected from the group consisting of a polyisocyanate (d.sub.1) and a compound (d.sub.2) which generates a polyisocyanate under heat, while at least two components (A) and (B) are melted. The impact-resistant polyacetal resin composition produced by this process gives injection-molded shaped articles showing highly well-balanced various physical properties such as tensile strength and elongation at break at weld portions, impact resistance and outer appearance than the conventional impact-resistant polyacetal resin composition.
The present inventors have newly found that (1) articles with weld portions, molded from the impact-resistant polyacetal resin composition produced by the process proposed above are required to be further improved in elongation at break at weld portions and (2) the impact-resistant polyacetal resin composition may be unsuitable as a molding material because its melt viscosity changes by elapse of time.